In most hand-held power tools a centrifugal clutch is interposed between a driving engine and the actual tool. This is the case where the engine is an internal combustion engine. At lower engine speed the centrifugal clutch is disengaged and the tool is inactive. When the driven part of the tool, e.g. a saw chain, is to be activated the engine rpm is increased and this increases the centrifugal force acting on a movable member of the centrifugal clutch and clutch engagement results.
Typically, such a centrifugal clutch has a driving disc, fixedly connected to the crank shaft via a hub, said driving disc having weighted arms or shoes. The centrifugal clutch further comprises a cup shaped clutch drum, surrounding the driving disc. The clutch drum is fixedly connected to a drum hub, which in turn is drivingly connected to a driven implement or tool. The cup shaped clutch drum is engaged by the weighted arms of the driving disc at higher engine speed and a rotational motion can be transmitted to the tool or implement via said drum hub.
For a hand-held power tool weight is a prime concern as the operator carries the tool during work. Every gram that can be saved is therefore important. Also the size of the tool is very important to make the tool more manoeuvrable and light. Therefore these tools are normally driven by compact and light two stroke engines.
Typically, the clutch drum is formed by drawing sheet metal with a thickness of approximately 1.8 mm. This thickness is used so that the peripheral portion can withstand the load from the weighted arms at maximum engine speed.
For a chainsaw the minimum width of the clutch drum is limited by a necessary minimum width of an outer cylindrical area of the peripheral portion contacted by a brake band. Commonly, the brake band is freely wound around the peripheral portion of the clutch drum so as to being able to instantly stop the clutch drum, and thereby also the chain, through friction, when the chain brake is actuated.
A disadvantage of the aforementioned clutch drum is its rather high weight and its large width.